Inductor for induction heating of metal strips

ABSTRACT

An induction heating device for continuously moving metal strip (22) for heat treatment or coating purposes comprises a flattened, spirally wound coil having the turns interlinked by an oblique connection on a small side of the coil, and surrounding the continuously moving metal strip (22). Means (24-26) are provided for suppressing the transverse component of the magnetic field created by the said coil.

BACKGROUND OF THE INVENTION

The present invention relates to treatments by a device for inductionheating of continuously moving metal strips for heat-treatment orcoating-treatment purposes; the coating may be metallic (reflow of tinor zinc) or organic (stoving of enamel or baking of paint). Inductionheating, because of its intrinsic qualities (cleanliness, rapidity,flexiblity) is a technique particularly well suited to the continuoustreatment of metal strips. This is why induction heating is employed forthe treatment of the coating, either metallic or organic coating, ofmetal strips, in particular steel strips.

In the case of the heating of magnetic steels, in particular, theinductors employed generate mainly a longitudinal magnetic fieldparallel to the direction of continuous movement of the metal strip. Theoperating frequency of the inductors depends on the electrical, magneticand geometrical characteristics of the metal strip, as well as on thepower density for magnetic materials. The inductors are constituted by acoil in the form of a flattened helix of several turns, supplied by afrequency converter.

Such an induction coil generates a magnetic field which includes asignificant longitudinal component, but unfortunately it also creates atransverse component. The first, longitudinal component generatesinduced currents which flow in the thickness of the stripperpendicularly to the direction of continuous movement and which heatit. The transverse component generates so-called leakage currents whichoriginate in the strip, initially travel along it in the direction ofcontinuous movement of the strip and which seek to return to the pointof departure, following all the pathways (roller, metal casing, etc.)possible.

If an insulator, for example a layer of paint or of enamel, isinterposed in the path of these leakage currents, a voltage thenappears, the level of which may cause the destruction of this insulator.This phenomenon, known by the name of the spark-erosion phenomenon, isparticularly detrimental to the quality of the paints or enamels whichare deposited on the metal strip.

As regards conductive coatings, that is to say metal coatings, there isno deterioration in the quality since the currents may flow freely; bycontrast, they may damage certain components, such as the bearings ofthe rollers for guiding the strip, for example. As regards organiccoatings, a highly insulated guiding system (roller mounted on a blockor coated with a plastic) may be used.

For multipurpose treatment installations, the use of metal rollers isessential. In this latter case, it is possible to place collectors onthe edges of the strip, upstream and downstream of the inductor, so asto localize the defects. Unfortunately, this system leads to themanufacture of a strip of lower quality since these edges are bared andthis is not a highly reliable system since there is the risk of "solidskin" paint breakdowns.

The origin of the leakage currents is the tranverse component of themagnetic field created by the inductor. The latter has, incross-section, the shape of an elongate rectangle surrounding the metalstrip to be treated and includes straight turns which are joinedtogether by an oblique connection arranged on the side of the turn.

It is desirable to eliminate the leakage currents in order to improvethe performance characteristics of the coating-treatment installationsand, in particular, to be able to use multi-purpose installations. Thisis why the present invention proposes to provide an inductor forinduction heating continuously moving metal strips, which enables theundesirable leakage currents to be eliminated.

SUMMARY OF THE INVENTION

For this purpose, the subject of the present invention is an inductorfor induction heating of continuously moving metal strips forcoating-treatment purposes, constituted by a coil in the form of aflattened helix, comprising turns connected together by an obliqueconnection arranged on a short side of the coil and surrounding thecontinuously moving metal strip, characterized in that means areprovided for suppressing the transverse component of the magnetic fieldcreated by the said coil.

Because the invention enables the transverse component of the magneticfield to be suppressed, the leakage currents disappear and theaforementioned draw-backs are no longer produced.

According to a first embodiment, the inductor includes a ringconstituting a short-circuited turn surrounding the short side of thecoil, including the oblique connections between turns.

This arrangement enables the effects created by the transverse componentof the magnetic field to be cancelled out.

According to another embodiment of the invention, the coil isconstituted by turns forming an interleaved double helix.

By virtue of this arrangement, the effects created by each of theoblique connections are eliminated in pairs.

According to yet another embodiment of the invention, the coil is madeup of two symmetrical coils having opposite directions of winding.

Here too, the deleterious effects of the oblique connections arecompensated for in pairs.

Other characteristics and advantages of the invention will emerge fromthe description which follows of exemplary embodiments of the invention,the description being given with reference to the appended drawings inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic perspective view of an installation for thecoating-treatment of metal strips, using induction heating;

FIG. 2 shows an induction coil of conventional type;

FIGS. 3, 4 and 5 illustrate a first embodiment of the invention;

FIG. 6 illustrates a second embodiment of the invention;

FIG. 7 illustrates a third embodiment of the invention, using two coilshaving opposite winding directions;

FIG. 8 represents a first way of connecting the coils of FIG. 7; and

FIG. 9 represents a second way of connecting the coils of FIG. 7.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 represents diagrammatically, in perspective, an installation forthe coating-treatment of a continuously moving metal strip, which usesan induction heating device. The metal strip 1 penetrates into theinstallation from the right-hand side and is conveyed as far as a guideroller 2 which may also constitute a cooling roller. Next, it passesthrough an induction heating assembly which, in the example represented,includes two successive induction coils 3 and 4 which are each suppliedby a frequency converter, respectively 5 and 6. The heated strip nextpasses into a treatment device shown diagrammatically by a bath 7.

FIG. 2 represents, in perspective, one of the induction coils. It ismade up of a metal strip which is wound so as to produce a flattenedhelix. In the example shown, the coil includes three turns which aresupplied with high-frequency current from the left-hand side, thecurrent leaving from the right-hand side, the flow of the current beingsymbolized by small arrows.

It is seen that each turn includes two long sides 11 and 12 and aperpendicular short side 13; the adjacent turns are joined together byan oblique connection 14. It is the set of these oblique connections 14which generates the transverse component of the magnetic field.

The invention proposes to cancel out this transverse component. A firstembodiment of the invention is illustrated by FIGS. 3 to 5. FIG. 3represents, in front section, an inductor 21 of the type which has justbeen described, which surrounds the metal strip 22. In FIG. 4, which isa section from above, and in FIG. 5, which is an end-on view, it is seenthat this inductor 21 includes four turns.

In accordance with the present invention, the short side 23 of theinductor 21, which includes the oblique connections between turns, issurrounded by a ring 24 which constitutes a short-circuit turn and whichhas an internal branch 25 located inside the inductor and an externalbranch 26. This short-circuit turn may, for example, be produced bymeans of a tube of rectangular cross-section, the thickness of which isgreater than the skin depth at the frequency used for the treatment.This turn 24 has a height such that it occupies the entire height of theinductor 21.

The face of this short-circuit turn, which is placed facing the turns ofthe inductor, in the region of the side where the connections betweenturns lies, is the site for induced currents of direction opposite tothe current of the inductor. The induced currents come together on theface facing the strip to be heated. The internal branch 25 does notconstitute a screen for the magnetic field; it regenerates thelongitudinal component of the magnetic field completely. The loop,constituted by the internal branch 25 and the external branch 26 and bythe set of connection bars 27 of the coil of the inductor, will consumeand cancel out, in the vicinity of the metal strip 22, the transversecomponent of the magnetic field. In fact, the loop, on the face facingthe turns and the connections, will be the site of an induced current inopposition with respect to the deleterious component of the current. Thestrength of the current flowing in this loop is a function of the mutualinductance between it and the set of turns and of connections. Theintensity may be adjusted by varying the geometry of the loop (distancebetween it and the set of connections and of turns); this adjustment maybe obtained by possibly using a magnetic circuit in order to increasethe coupling. It is thus possible to obtain a state in which thetransverse component of the field in the region of the strip 22 is zero.If this turns out to be necessary, the short-circuit turn 24 will becooled, for example by water.

FIG. 6 represents another embodiment of the invention. In this case, themetal strip 31 moves in an inductor constituted by an interleaved doublehelix. The two helices include turns joined by oblique connections andthe second helix is wound in the reverse direction to that of the firstin such a way that the output terminal of the second helix lies in theregion of the input terminal of the first.

It is seen that the oblique connections of the first helix cross overthe oblique connections of the second helix in such a way that thetransverse component of the magnetic field is cancelled out.

In fact, it is possible to consider that each turn is constituted by alength of the first helix and a length of the second helix. Taking intoaccount the symmetries necessary in this arrangement, the number ofturns is equal to 4n+1, n being an integer. If a slight asymmetry istolerated, it is possible to depart from the 4n+1 condition; however, itwill be necessary for the number of turns to be large (for examplegreater than 7).

FIG. 7 represents a third embodiment of the invention, in which twocoils arranged one after the other are used. A first coil 41 is wound ina first direction and the second coil 42 is wound in the reversedirection. These two coils may be connected in series or in parallel.

FIG. 8 represents the series mounting of the two coils 41 and 42 of FIG.7. The connection 43 produced is made so that the inducing currents ofeach coil rotate in the same direction and therefore generatelongitudinal fields in the same direction. The terminals 44 and 45 arejoined respectively to the poles of a source of alternating current.

From the electrical standpoint, the inductor of FIG. 8 is equivalent toan inductor of conventional type which includes twice the number ofturns of each elementary coil 41 or 42.

FIG. 9 represents a parallel connection of the two coils of FIG. 7. Inthis case, the middle parts and the end parts of the coils areinterconnected, the supply terminals 51 and 52 being connected to thealternating-current supply.

The current supplied by the continuous supply source is divided into twocurrents, each travelling along one of the two coils. In order to havemaximum efficiency, it is advantageous to balance the fields created byeach of the coils in terms of both phase and amplitude. In the case ofcoils produced by means of conductive strips, the equality of the fieldscreated by the two coils is relatively easy to obtain. If necessary, inthe case where the currents of the two coils are markedly different, itis possible to act on the structure of a coil by varying the number ofturns or the length of them.

According to another variant, it is possible to use current-correctingdevices, for example impedances, in each coil 41 and 42.

It is seen that the invention makes it possible to suppress thedeleterious effects of the transverse-field component of a heatinginductor of known type. The various embodiments of the invention areeasy to produce and they make it possible to obtain an induction heatinginductor which can be used for any type of coating treatment, namelymetal or organic coating treatment.

What is claimed is:
 1. A longitudinally extending inductor (3,4), forinduction heating of continuously longitudinally moving metal strips(1,22,31) for heat-treatment or coating-treatment purposes, constitutedby a coil in the form of a flattened helix, said coil comprising: aplurality of turns (11-13) connected together by oblique connections(14) arranged on a short side of the coil and surrounding eachcontinuously moving metal strip (1,22,31); and means (24-26) forsuppressing a transverse component of the magnetic field created by saidcoil.
 2. Inductor for induction heating of continuously moving metalstrips according to claim 1, wherein said means includes a ring (24)constituting a short-circuited turn surrounding the short side (23) ofthe coil (21), including the oblique connections between turns. 3.Inductor for induction heating of continuously moving metal stripsaccording to claim 2, wherein the said ring (24) extends oversubstantially the entire height of the coil (21).
 4. Inductor forinduction heating of continuously moving metal strips according to claim2, wherein the ring (24) is constituted by a tube, the thickness ofwhich is greater than a skin depth for the frequency of the inductioncurrent.
 5. Inductor for induction heating of continuously moving metalstrips according to claim 2, 3, 4, or 7, wherein said ring (24)surrounds connection bars (27) of the coil.
 6. Inductor for inductionheating of continuously moving metal strips according to claim 1,wherein the coil is constituted by turns forming an interleaved doublehelix.
 7. Inductor for induction heating of continuously moving metalstrips according to claim 3, wherein the ring (24) is constituted by atube, the thickness of which is greater than a skin depth for thefrequency of the induction current.
 8. The inductor according to claim1, wherein said coil has in cross-section the shape of an elongatedrectangle which surrounds the metal strip.